LOS ANGELES  Beach sand can be teeming with bacteria even when the ocean water is clean, according to a study released on Tuesday.

Health officials have long known that urban runoff pollutes ocean water with microbes including E. coli and enterococci bacteria found in fecal material. The study by University of California, Los Angeles researchers found microbes can grow in the sand as well, and remain there long after the ocean has flushed itself clean.

"Even on days when the water is very clean, bacteria is still in the sand for a week," said Jennifer Jay, a UCLA environmental engineering professor who headed the study. "We feel it can be an important exposure route" for contamination.

Jay and a graduate researcher tested three beaches -- Surfrider Beach in Malibu, Santa Monica Beach and Mother's Beach in Marina del Rey -- during a storm in February 2003. They also surveyed sand at 13 Santa Monica Bay beaches from Malibu to Redondo during the summer, focusing on wet sand near the water's edge.

They found that sand bacteria concentrations at sheltered beaches favored by parents with toddlers were 1,000 times higher than at beaches that were open to the ocean.

However, Jay said it's hard to evaluate the health risk these bacteria pose because health standards for beach sediment have not been developed.

Researchers will next examine whether viruses are present in the sand, she said.

Mark Gold, executive director of Santa Monica-based Heal the Bay, said the study adds urgency to beach cleanup efforts and could lead to more health warnings posted along the shore.

Heal the Bay on Wednesday plans to release a list of the most polluted California beaches.